1. Field of the Invention
This invention relates to optical fibres, and in particular, to the control of the optical polarisation properties of optical fibres by propagation of acoustic waves therein.
2. Description of the Related Art
The propagation of electromagnetic radiation in optical fibres is well understood, and has particular applications in the areas of communications and measurement sensing. Information may be conveyed along the fibre by modulating the light in one of several ways: the modulation may be in the form of a variation in the amplitude, phase, frequency or polarisation state of the propagating light. If the modulation is performed at one end of the fibre and is detected and demodulated at the other end, the fibre may be used as a communications medium; if the modulation is effected by means of a field external to the fibre while the light is propagating within it, the emergent light may be detected and demodulated to allow measurement of the external field, and the fibre then comprises a measurement sensor.
It is frequently desirable to control the polarisation properties of optical fibres. In communications this may be either to limit the effects of polarisation dispersion or to increase the efficiency of coherent detection. More often, the control is required for measurement sensors, since the polarisation properties of, especially, monomode optical fibres are very sensitive to external influences, and thus feature strongly in measurement functions.
Static control of polarisation properties is provided by fabricating fibres with special geometries or special strain distributions. The present invention provides the means for simultaneous time-variable and position-variable control of the polarisation properties from any position along the length of the fibre, which may be conveniently one or other of the two ends.